Electronic distance measurement (EDM) units are used to measure distance to a target. In time-of-flight distance measurement, the time delay between emission and reception of a laser pulse allows for distance calculation. The emitted pulse travels to a reflector and back to a detector where it is received. The time delay is proportional to the pulse travel distance.
For a given reflector, the power level of the received pulse is assumed to be inversely proportional to the square of the pulse travel time (and the pulse travel distance) within a range of interest. The dynamic range of the power level of received pulses thus can be quite large.
To accommodate this dynamic range, prior art systems apply an attenuation function such that detection sensitivity increases over time following emission of a send pulse.
Typical prior art systems use an attenuation function based on a fixed function with fixed timing relative to the send pulse. U.S. patent application Ser. No. 13/597,295 filed 29 Aug. 2012 describes systems in which at least one of an attenuation function and an offset of the attenuation function relative to the send pulse is variable.
EDM units are used in surveying instruments such as total stations and stationary and mobile scanners. Total stations typically make distance measurements at a lower repetition rate than scanners, so that time is available to determine the amplitude of a return pulse and to set the attenuation of the detector circuitry.
Returning pulse amplitude is determined, for example, by sending a first pulse whose returning pulse amplitude is used to set the attenuation for the returning pulse of a subsequently emitted pulse directed at the same target. In some instruments a grey-wedge optical attenuator is adjusted over multiple measurement attempts until the return pulse signal is within a fixed receiver detection corridor.
Another approach is to set the attenuation level by predicting amplitude of a returning pulse based on prior distance measurements of nearby target locations, as described in WO 2008/089788 A1.
A further approach, described in WO 2009/039875 A1, is to split the return pulse signal, using one portion to set attenuation and delaying the other portion so that its amplitude is measured after the attenuation has been set.
Still, surveying instruments with improved compensation are desired to obtain a faster scanning and/or a more adapted dynamic range.